Radio communication units have been proposed, having one transceiver for communicating over, say, a satellite network,and a second transceiver for communicating over a short range radio link with a remote unit such as a portable handset.
A potential problem with such units is that, if there is a third party transmitter located close to the unit, or to the remote handset, operating in a nearby frequency band, it may together with the transmitter on the short range link produce intermodulation products which can interfere with reception of signals on the long range communications link. Typically, the short range radio link uses a frequency hopping system, in which communications are transmitted over a large number of channels, which are used in sequence. It is probable that only one of these channels will, together with the third party signal, produce an interference signal on the channel used by the long range radio link.
Various systems are known for avoiding interference. For example, EP-0781066 describes an arrangement in which interference signals are monitored on a number of channels, and channels with unacceptably high levels of interference are not used. WO95/08246 discloses a frequency hopping system, in which interference levels are monitored on the available channels and, if it is determined that the interference level on a channel is high, this channel can be discarded from the frequency hopping sequence which is used.
Thus, these systems involve detecting interference on a particular channel and, when interference is detected, avoiding the use of that channel.